Stitch together and enhance phone-photos of old letters

I have recently had access to old letters that are of value to my family. These letters are from the 1980's and include post cards and hand written letters. I only had access to them for a limited time so took photos of the letters with the camera on my phone (Google Nexus phone). For clarity I took lots of photos and for letters often took 4-5 photos of an A4 page rather than one from a distance to get the whole page in. I would like to turn these images into a physical album so I need the letter photos cropped, stitched together and enhanced where needed so they can be read. There are perhaps 500 images but each letter has between 8 and 10 photos of it so it is around 50 letters or post cards in total. Attached is a sample of 15 images that create one of the letters (the first is the front of the envelope, the back is the sent from address) so these images need to be stitched together and enhanced as needed to make one letter.